In recent years, from the viewpoint of reducing CO2 emissions as well as further advancing energy conservation, there has been demand for transportation devices including automobiles and motorcycles having a vehicle body with reduced weights, thereby reducing the energy consumption. As part of the weigh reduction of the vehicle body, the size or weight of mechanical components such as gears and shafts has been reduced. This leads to the fact that these mechanical components are required to have improved wear resistance or fatigue resistance.
Conventionally, the wear resistance or fatigue resistance of these mechanical components such as gears has been usually improved by applying case-hardening processes typified by quenching processes using carburizing and nitriding. However, in the case of mechanical components which have case-hardened processes applied thereto with the aim of achieving improved smoothness and quietness thereof during operation, in order to respond to technical demands for improvements in the accuracy of the dimensions of these mechanical components, it is extremely important to reduce distortion occurring during the case-hardening processes (hereinafter, also referred to as thermal treatment distortion) as much as possible.
As for a measure for reducing the thermal treatment distortion, Patent Documents 1 and 2 disclose the example of a method of adjusting internal structures so as to have an austenite+ferrite layer after a thermal treatment using carburizing and nitriding, and quenching the steel, thereby manufacturing a high-strength gear having reduced distortion.
However, with this method, resistance to softening is low due to a small amount of Si in the steel used. This leads to a problem in that, in an application where the produced gear is used at a high rotational speed, temperatures on the surface increase, and the surface is softened, whereby pitting resistance reduces.
Patent Document 3 discloses a case hardened steel having thermal treatment distortion reduced in a similar manner. However, this case hardened steel has a large amount of C, and hence, has a problem of deterioration in machinability, cold working characteristics, toughness or other characteristics.
Patent Document 4 discloses a steel for gears, in which an ideal critical diameter after carburizing processes is defined, and an inner portion of the metal where carburizing and nitriding are not applied after carburizing and quenching has a carburized and quenched structure having reduced distortion with ferrite: 10 to 70%. However, this steel for gears has a problem of deterioration in characteristics related to carburizing due to a large amount of Si, and deterioration in machinability and a cold working characteristic.
Patent Document 5 discloses a method of reducing the thermal treatment distortion by appropriately adjusting chemical components in steel, and employing appropriate conditions for carburizing processes. Further, Patent Document 6 discloses a method of reducing distortion after thermal treatments by controlling critical cooling rates using the amount of C or the amount of Mn in steel.
Patent Documents 7 and 8 disclose a method of applying quenching processes after case-hardening processes by setting a quenching starting temperature depending on chemical components, thereby adjusting an area fraction of pro-eutectoid ferrite in a structure of a core portion after the case-hardening process, in other words, a structure of a non-carburized layer so as to fall in the range of 20 to 80%.
Patent Document 9 discloses a method of reducing an amount of distortion by applying processes of carburizing, cooling, reheating, and quenching, thereby reducing the thermal treatment distortion and improving bending fatigue strength. However, with this method, it is not possible to prevent deterioration in productivity and increase in costs of thermal treatments resulting from reheating and quenching.
Patent Document 10 discloses a steel for nitriding that does not have any substantial white band, in which pressure is applied to unsolidified regions under specific conditions, electro-magnetic stirring is not performed at solidification end positions so as not to generate any white band, the degree of segregation C/Co at a D/4 portion is set so as to fall in a range of 0.99 to 1.01.
Patent Document 11 discloses a case hardened steel in which a difference between the maximum value and the minimum value of the degree of micro-segregation of C and Mn in a cross section of bloom in a radial direction is not more than 0.03%, and a difference in contents adjacent to each other is not more than 0.02%. Further, Patent Document 12 discloses a case hardened steel having reduced distortion and manufactured from a bloom having a degree of segregation of C at the center in a range of 1.1 to 1.0.
However, in reality, any of the methods and steels described above cannot achieve the reduction in distortion that satisfies the severe demands made by recent consumers.